Device for measuring amount of displacements with aid of gratings

ABSTRACT

A device for measuring the magnitude of displacements with the aid of gratings having a real image optical system whose magnification is one with a reflection optical system adapted to reflect a light incident on a grating plane and project the light again on the same grating plane in a direction which is the same as that of the incident light to produce a moire fringe whose strength varies in response to the displacement of the grating. The moire fringe thus produced is observed to measure the amount of displacement of the grating. An elongated grating is used in case of measuring a linear displacement of the grating, while a radial grating is used in case of measuring an angular displacement of the grating.

I United States Patent 11113523370 [72] Inventor Yoshisada Hayarnizu[56] References Cited Tokyo, J 111ml FOREIGN PATENTS 1 APP 782,831 1957GreatBritain .356/169 22] Filed J 991,710 1965 GreatBritain 250/2370[45] Patented Dec.2l,1971

ol 0 ti lc mm d 398,097 1966 Switzerland... 356/l69 1 Asslsneei im? P m782,831 1957 GreatBritain..... 356/169 i gfga 991,710 1965GreatBritain.. 250/2370 [32] 398,097 1966 Switzerland 356/l69 [33] Japan[31] 42/17363; Primary Examiner- Ronald L. Wibert Sept. 20, 1967, Japan,No. 42/598139 Assistant Examiner-Jeff Rothenberg Continuation ofapplication Ser. No. Attorney-Waters, Roditi & Schwartz 697,175, Jan.11, 1968, now abandoned. This application July 2, 1970, Scr. No. 56,114ABSTRACT: A device for measuring the magnitude of displacements with theaid of gratings having a real image optical system whose magnificationis one with a reflection optical [54] DEVICE FOR MEASURING AMOUNT OF Isystem adapted to reflect a light incident on a grating planeDISPLACEMENTS WITH AID 0F GRATINGS and project the light again on thesame grating plane in a 5 Claims,9Drawing Figs. direction which is thesame as that of the incident light to produce a moire fringe whosestrength varies in response to [52] U.S.Cl 2 the displacemem of he g gThe moire fringe thus [51] ll cl Golb 11/26 produced is observed tomeasure the amount of displacement [50] Field 0 Search 250/237 of thegrating. An elongated grating is used in case of measur- 356/l39 152 169170 171 ing a linear displacement of the grating, while a radial gratingis used in case of measuring an angular displacement of the grating.

PATENTED BEBE! 1911 SHEET 1 0F 4 PATENIED UEEZi 197a SHEET 2 BF 4PAIENIEMEEN 3,62 ,8T

SHEET 3 BF 4 I DEVICE FOR MEASURING AMOUNT F DISPLACEMEN'IS WI'IIIAID 0FGRA'IINGS This is a continuation-of application Ser. No. 697,175, filed.Ian. ll, l968,now abandoned.

The present invention relates to a device for measuring the magnitude ofdisplacements with the aid of gratings and more particularly a devicefor measuring the magnitude of a linear displacement by means of anelongated grating and the magnitude of an angular displacement by meansof a radial grattng. v

Heretofore, it has been proposed to cause one or two gratings to producea so-called moire fringe and observe the variation of the strength ofthe moire fringe caused by the displacement of the grating so as tomeasure the displacement of the grating. However, such well-knowndevices require a high degree of manufacturing techniques andtolerances, are low in measuring accuracy, and are not stable inoperation, so that they are very inconvenient in case of applying themto the practical field for measuring displacements. v

' The principal objectof the invention is to obviate such disadvantagesand provide a device for measuring linear and angular displacements byutilizing the moire fringe, and which is easy in assembling and high inaccuracy and extremely stable in operation.

A feature of the invention is the provision of such an improved devicefor measuring the magnitude of displacements with the aid of gratings,which incorporates a real image optical system whose magnification isone, said optical system including a reflection optical system adaptedto reflect a light incident on a grating plane so as to cause thereflected light to project again onto the grating plane in a directionwhich is the same as that of the incident light and being capable ofplacing an image of the grating upon the grating plane and henceproducing a moire fringe whose strength varies in response to thedisplacement of the grating and hence measuring the amount ofdisplacement of the grating.

Other objects, features and advantages of the invention will becomeapparent from a consideration from the following description inconjunction with the accompanying drawing, in which:

FIGS. 1, 2 and 3 are diagrammatic illustrations of devices for measuringdisplacements with the aid of gratings heretofore proposed;

FIGS. 4 and 5 are diagrammatic illustrations of an embodiment of adevice for measuring displacements with the aid of gratings according tothe invention with an elongated grating;

FIG. 6 is a diagrammatic illustration of another embodiment of thedevice according to the invention with a radial grating;

FIG. 7 is a diagrammatic illustration of a modified form of a part ofthe device shown in FIG. 6;

FIG. 8 is a plan view of the device shown in FIG. 7; and

FIG. 9 is a side elevation of the device shown in FIG. 7.

The invention will now be described with reference to devices formeasuring the magnitude of displacements with the aid of gratingsheretofore proposed and shown in FIGS. 1-3.

A device shown in FIG. 1 comprises two gratings A and B arranged inopposition and closely spaced apart by a distance d. A light I, incidenton the two gratings A and B emits light 1, whose strength varies inresponse to the relative position of the two gratings A and B. Thestrength of the emitted light I, varies simultaneously when the gratingB is displaced in a direction shown by an arrow x which is substantiallyperpendicular to the direction of the grating lines shown by dottedlines. This sinusoidally varying light I, arrives at a photoreceiver pand electric pulses generated can be counted so as to measure thedisplacement of the grating B.

Another device shown in FIG. 2 comprises two gratings A and B and a realimage lens system L whose magnification is one. This device has theadvantage that the value corresponding to the distance d shown in FIG. 1becomes almost zero,

of the light I, leaving the grating B.

n A still further device shown in FIG. 3 comprises one grating G and areflection optical system R and is capable of placing the image of thegrating G reflected at the reflection optical system R upon the gratingplane G. This device has the advantage that the displacement of thegrating G which is onehalf that of the grating shown in FIG. 1 issufl'tcient to produce electric pulses which are the same in number asthose produced by the device shown in FIG. I provided the gratingconstants are the same in both devices and hence the accuracy becomeshigher.

However, all of the above devices require a high degree of manufacturingtechniques, are low in measuring accuracy, and are not stable inoperation for the following reasons.

In the device shown in FIG. I it is required to make the value of thedistance d extremely small so that an elongated grating could not beutilized. Moreover, the moire fringe produced is very sensitive toslight variations of the relative position between the grating lines ofboth of the stationary grating A and the movable grating B. Thus, it isnecessary to control the direction of the grating line in an extremelysevere manner in case of displacing the grating B.

The device shown in FIG. 2 is required to displace the grating by anextremely small distance along the direction of the grating line andplane. In order to obviate such disadvantage, the gratings A and B maybe assembled into an integral unit. In this case, however, the gratingsA and B should be made the same in length and the two elongated gratingsA and B should be displaced whilemaintaining a great distancetherebetween, with the result that the space necessary for thedisplacement of the two elongated gratings spaced apart by the greatdistance becomes very large, that the device as a whole becomesconsiderably large in size, and'that in case of A building the device ina milling machinethe device becomes located upon a mechanism necessar-for the milling machine so that such built-in device hinders theoperation of the milling machine and hence could not be employed for it.

The device shown in FIG. 3 makes use of one grating only so that itsaccuracy becomes two times higher than that of the devices shown inFIGS. I and 2. But, this device has the disadvantage that the gratingplane should be displaced by one-half less than the displacement of thedevices shown in FIGS. I and 2. If the grating plane is displaced to apoint p shown in FIG. 3, the image of the point P, will be reproduced ata point P, and hence will not be placed on the grating plane. Themagnification of the reflection optical system R is one so that thedisplacement of the grating plane causes the image to displace two timesgreater than the displacement of the image reproduced by the devicesshown in FIGS. 1 and 2. Thus, the device shown in FIG. Spermitsdisplacing the grating plane for a distance which is one-half that ofthe devices shown in FIGS. 1 and 2 in order to obtain the same degree ofthe displacement of the image, with the result that the device shown inFIG. 3 requires an extremely high accuracy. Moreover, the device shownin FIG. 3 has the disadvantage that provision must be made for asemitransparent mirror which limits the field necessary for the gratingplane and hence the brightness thereof becomes decreased.

In order to obviate the above disadvantages of the heretofore-proposeddevices, the invention (FIG. 4) provides such an improved device whichmakes use of a suitable inverted real image system whose magnificationis one and including reflection mirrors M,, M,, M, and M convergentlenses L, and L and an elongated grating G. These optical elements ofthe real image system are constructed and arranged such that light I,incident on the grating G in a direction which is substantiallyperpendicular thereto is reflected at the mirrors M, and M, to adirection which is substantially opposite to the direction of theincident light I, and then passes through the lens L, and then reflectedat the mirror M, and the reflected light passes through the lens L, andarrives at the mirror M, where it is reflected in a direction which issubstantially the same as that of the incident light I,. The convergentlenses L, and L, consist of lens systems having the same construction.It

the focal points of the lenses I. and L are located on the grating planeG, these lenses are capableof inverting the position of the image of thegrating from upside to down and vice versa to place the image of thegrating upon the grating plane and provide an inverted real image systemwhose magnification is one.

If it is assumed that the image of a point P on the grating plane isreproduced at a point I and the grating G is caused to displace in adirection x shown by an arrow, the image of that portion of the gratingwhich is positioned near the point P and reproduced near the point Pwill be moved relative to that portion of the grating which ispositioned near the point P with a speed which is two times higher thanthe displacement of the grating G. Thus, the strength of the moirefringe produced by placing the image upon the grating variessinusoidally in response to the displacement of the grating G, therebygenerating a number of electric pulses at a photoreceiver p. The numberof these pulses are two times larger than that of the grating linescorresponding to the amount of displacement of the grating G, with theresult that the displacement of the grating G can be measured bycounting the variation of the strength of the moire fringe in anaccurate manner, the accuracy being two times higher than that of the vwell-known devices.

An image Q of a point Q not located on the grating plane G is notplacedupon the grating plane G as shown in FIG. 5. In such case, the realimage optical system whose magnification is one makes it possible toalways satisfy the relation QP=QP Thus, if the grating face is displacedfrom a plane containing PP to another plane containing (10', the imageof the grating will be placed upon the grating plane. Thus, the contrastof the moire fringe is never deteriorated. The device shown in FIG. 4renders it possible to effect the measurement without adverselyaffecting it even when the grating is displaced in a directionperpendicular to the grating plane.

If the grating rotates during the displacement thereof owing tomechanical deficiency or the like, the image of the grating will alsorotate in a direction which is the same as that of the grating so as tomake the difference in directions of the grating and the image thereofalways constant. This is evident, because the image of the grating as awhole is reproduced by means of the inverted real image system whosemagnification is one, so that the position of the image is inverted fromupside to down and vice versa.

If the difference in the directions of the grating and of the imagethereof varies, the strength of the moire fringe will also be changed inan extremely sensitive manner, so that it is necessary to make suchdifference always constant. The invention permits maintaining suchdifference always constant even when the grating is inclined from itsoriginal position, so that special attention is not required.

As can be seen from the above, the device according to the inventionincorporates a real image optical system whose magnification is one,said optical system including a reflection optical system adapted toreflect light incident on a grating plane so as to cause the reflectedlight to project again onto the same grating plane in a direction whichis the same as that of the incident light and is capable of placing animage of the grating upon the grating plane and producing a moire fringewhose strength varies in response to the displacement of the grating andhence measuring the amount of displacement of the grating. The deviceaccording to the invention has the advantage that the displacement ofthe grating can be measured with high accuracy in an extremely stablemanner without being adversely affected by the displacement of thegrating plane and by the inclination of the grating plane from itsoriginal position.

In the above-mentioned embodiment, use is made of an elongated gratingso as to measure its linear displacement, yet the principle according tothe invention may also be applied to a radial grating so as to measureits angular displacement with a high accuracy and in a stable manner.

FIG. 6 diagrammatically illustrates such an improved device according tothe invention with a radial grating.

jective lens, 10 a third rectangular prism, 11 a condensing lens, 12 aphotoelectric element, and G a radial grating.

FIGS. 7-9 shown another embodiment of the invention in which the samereference numerals are used as in FIG. 6 but a parallelogram prism 14 isused in place of the first and second reflecting mirrors 6 and 7. Theeffect of the reflecting plane of this prism 14 is the same as that ofthe reflecting mirrors 6 and 7 shown in FIG. 6.

Light leaving the light source 1 arrives at the collimator lens 2 whereit becomes parallel light which is projected on a portion A of theradial grating G. The light leaving the portion A passes through anoptical system including the first rectangular prism 3 and thecondensing lens 11 and hence passes again through the plane of theradial grating G. The important feature of the invention is that in thiscase the light is reflected several times and projected onto the gratingG in a direction which is the same as that of the incident light.

The optical path of the above-mentioned optical system will now bedescribed with reference to FIGS. 6-9. The light incident on the gratingplane G in a direction perpendicular thereto is reflected at the firstrectangular prism 3 to a direction which is at right angles to theincident ray and passes through the center of the first objective lens 4and is reflected two times at the pentagonal prism 5 to a directionwhich is at right angles to the light leaving the first objective lens4. The light leaving the pentagonal prism 5 is reflected at the firstand second reflecting mirrors 6 and 7 shown in FIG. 6 or at theparallelogram prism 14 shown in FIGS. 7-9 and projected onto the secondrectangular prism 8 where the light is reflected to a direction which isat right angles to the incident light and passes through the center ofthe second objective lens 9 and reflected at the third rectangular prism10 to a direction which is at right angles to the light arriving at theprism 10 and perpendicular to the plane of the grating 13, that is, adirection which is the same as that of the incident light.

The first and second objective lenses 4 and 9 are assumed to be equal infocal distances thereof and are assumed to have focal planes which arecoincident with the radial grating plane G.

The optical system included in the path from the first rectangular prism3 to the condensing lens I] constitutes a real image optical systemwhose magnification is one and the pentagonal prism 5 acts as an opticalsystem for inverting the position of the image of the grating G fromleft to right and vice versa.

Thus, an image of a portion A of the grating 13 is placed upon a portionB of the grating 13. The radial grating G has a sector form which isdivergent outwards from the center thereof so that it is necessary toinvert the position of the image of the grating from left to right andvice versa. If the position of the image of the grating G is invertedfrom top to bottom in the radial direction the image of the portion Acould not be placed upon the portion B of the grating G. Thus, in thisembodiment it is impossible to use an inverted real image opticalsystem.

The above-mentioned real image optical system adapted to invert theposition of the image of the grating from left to right and vice versaand including the optical elements arranged in the optical path from thefirst rectangular prism 3 to the third rectangular prism 10 makes itpossible to place the image of the portion A of the grating plane uponthe portion B thereof to produce the moire fringe. The contrast betweenthe lightest and darkest areas in the moire fringe varies in response tothe angular displacement of the grating G. The condensing lens 11 servesto project the image of the portion B where the moire fringe of thegrating 13 is produced onto the photoelectric surface of thephotoelectric element 12. The photocurrent varies in dependence with thedegree of the contrast of the moire fringe. Thus, it is possible tomeasure a sequence of degrees of the contrast by counting the variationsof the photocurrent or by counting the rotating angle of the radialgrating plane.

Thus, the device according to the invention comprising a real imagesystem whose magnification is one and adapted to project again anincident light on the grating plane in a direction which is the same asthat of the incident light is capable of always placing the image on theportion A of the grating 13 upon the portion B even when the gratingplane moves in a direction perpendicular to the plane thereof as shownby arrows a in MG. 7 and hence of substantially preventing defocusingphenomenon.

Thus, the contrast of the moire fringe does not change even if thegrating plane of the radial grating G is moved in a directionperpendicular to the grating plane or is inclined at a certain anglefrom the original position.

The important feature of the device according to the invention is thatthe device is easy in assembling and stable in operation.

The invention makes use of a real image system whose magnification isone and adapted to invert the position of the image of the grating fromleft to right and vice versa. This real image system causes thephotoelectric element 12 to produce a number of pulses which are twotimes larger than the number of gratings corresponding to the amount ofthe angular displacement of the grating plane G. Thus, the invention hasthe advantage that the angular displacement of the grating plane G canbe measured with an accuracy which is two times higher than that of thewell-known device or the total numbers of the gratings on the grating Gcan be reduced by half.

The inversion of the position of the image of the grating I from left toright and vice. versa means that the peripheral direction of a smallline portion of the radial grating plane is inverted by means of anoptical system to produce an inverted image whose peripheral directionis opposite to that of the small line portion.

The above-mentioned real image optical system whose magnification is oneand adapted to invert the peripheral direction of the image of thegrating from left to right and vice versa is assumed to be an opticalsystem in which an object on the grating plane of the rotatable radialgrating and the image of such object are symmetrical with respect to aplane containing a rotating axis of the grating.

As above stated, the device for measuring angular displacement with theaid of a radial grating according to the invention has the advantagethat the assembling and adjustment can be effected in an extremely easymanner, that the device as a whole is stable against the movement of thegrating plane in a direction perpendicular to the grating plane and theinclination of the grating plane etc. from the original positionthereof, that the measurement can be effected with a high accuracy, andthat the number of grating lines can be reduced by half.

It will be appreciated that the invention is not restricted to theembodiments described and that many variations are possible to a personskilled in the art without departing from the scope of the invention.For example, a porros mirror optical system based on the principle ofthe first kind of porros prism may be used. In this case, it isnecessary to form the inverted real image optical system whosemagnification is one by combining the porros mirror optical system withan erect real image optical system whose magnification is one.

iclaim:

1. A device for measuring the magnitude of displacement of diffractiongratings, which comprises a light-transmitting diffraction grating and areal image optical system whose magnification is unity, and including alight source, objectives a reflection optical system and a photocell inan arrangement such that light from the light source is initiallyprojected through the grating in a direction substantially perpendicularthereto, is second directed substantially parallel with the lines of thegrating, is third directed in a direction opposite to that of theinitially projected light, is fourth directed again in parallel with thegrating but in a direction opposite to that of the second direction, andfinally again directed through the grating in the same direction as theinitially projected light onto said photocell whereb to form asubstantially closed and rectangular-shaped h t passage, said real imageoptical system including optical elements all of which, inclusive ofsaid light source andphotocell, are arranged in sequence along saidlight passage, said objectives and reflection optical system producingon said grating an image of the portion of the grating illuminated bysaid initially projected light such that a moire fringe pattern isproduced by the superposing of the image upon the grating, said imagebeing left to right reversed and slightly angularly offset relative tosaid grating so as to produce said moire fringe pattern, which fringepattern can be detected by said photocell to measure the amount ofdisplacement of the grating.

2. A device as claimed in claim 1 wherein said objectives and reflectionoptical system include a first rectangular prism for receiving lightfrom said source, a first objective lens positioned to receive lightfrom said prism, a pentagonal prism in the light path from said lens, afirst reflecting mirror for reflecting light received from saidpentagonal prism in said second direction, a second reflecting mirrorfor reflecting light from the first mirror in said third direction, asecond rectangular prism for receiving light from the second mirror, asecond objective lens for receiving light from the second prism and acondensing lens for transmitting and concentrating light from saidsecond lens to said photocell, said grating being an angularlydisplaceable radial grating interposed between said first rectangularprism and said third rectangular prism.

3. A device as claimed in claim 1 wherein said objectives and reflectionoptical system include a first rectangular prism for receiving lightfrom said source, a first objective lens positioned to receive lightfrom said prism, a pentagonal prism in the light path from said lens aparallelogram prism for refracting light from said pentagonal prism insaid second direction, a second rectangular prism for receiving lightfrom the para]- lelogram prism, a second objective lens for receivinglight from the second prism and a condensing lens for transmitting andconcentrating light from said second lens to said photocell, saidgrating being an angularly displaceable radial grating interposedbetween said first rectangular prism and said third rectangular prism.

4. A device as claimed in claim 1 wherein said objectives and reflectionoptical system include a first reflecting mirror for reflecting an imageof said grating illuminated by said source in said second direction, asecond reflecting mirror for reflecting the image from said first mirrorin said third direction, a first objective lens in the light passage insaid third direction, a fourth reflecting mirror for receiving the imagefrom the first objective lens and reflecting the image in said fourthdirection, a second objective lens in the path of the image in saidfourth direction, and a fifth reflecting mirror for reflecting the imageto said photocell via said grating in the final direction and producingthe moire fringe pattern.

5. A device as claimed in claim 4 wherein said grating is an elongatedplanar grating adapted to measure the linear displacement of thegrating.

1. A device for measuring the magnitude of displacement of diffractiongratings, which comprises a light-transmitting diffraction grating and areal image optical system whose magnification is unity, and including alight source, objectives a reflection optical system and a photocell inan arrangement such that light from the light source is initiallyprojected through the grating in a direction substantially perpendicularthereto, is second directed substantially parallel with the lines of thegrating, is third directed in a direction opposite to that of theinitially projected light, is fourth directed again in parallel with thegrating but in a direction opposite to that of the second direction, andfinally again directed through the grating in the same direction as theinitially projected light onto said photocell whereby to form asubstantially closed and rectangular-shaped light passage, said realimage optical system including optical elements all of which, inclusiveof said light source and photocell, are arranged in sequence along saidlight passage, said objectives and reflection optical system producingon said grating an image of the portion of the grating illuminated bysaid initially projected light such that a moire fringe pattern isproduced by the superposing of the image upon the grating, said imagebeing left to right reversed and slightly angularly offset relative tosaid grating so as to produce said moire fringe pattern, which fringepattern can be detected by said photocell to measure the amount ofdisplacement of the grating.
 2. A device as claimed in claim 1 whereinsaid objectives and reflection optical System include a firstrectangular prism for receiving light from said source, a firstobjective lens positioned to receive light from said prism, a pentagonalprism in the light path from said lens, a first reflecting mirror forreflecting light received from said pentagonal prism in said seconddirection, a second reflecting mirror for reflecting light from thefirst mirror in said third direction, a second rectangular prism forreceiving light from the second mirror, a second objective lens forreceiving light from the second prism and a condensing lens fortransmitting and concentrating light from said second lens to saidphotocell, said grating being an angularly displaceable radial gratinginterposed between said first rectangular prism and said thirdrectangular prism.
 3. A device as claimed in claim 1 wherein saidobjectives and reflection optical system include a first rectangularprism for receiving light from said source, a first objective lenspositioned to receive light from said prism, a pentagonal prism in thelight path from said lens, a parallelogram prism for refracting lightfrom said pentagonal prism in said second direction, a secondrectangular prism for receiving light from the parallelogram prism, asecond objective lens for receiving light from the second prism and acondensing lens for transmitting and concentrating light from saidsecond lens to said photocell, said grating being an angularlydisplaceable radial grating interposed between said first rectangularprism and said third rectangular prism.
 4. A device as claimed in claim1 wherein said objectives and reflection optical system include a firstreflecting mirror for reflecting an image of said grating illuminated bysaid source in said second direction, a second reflecting mirror forreflecting the image from said first mirror in said third direction, afirst objective lens in the light passage in said third direction, afourth reflecting mirror for receiving the image from the firstobjective lens and reflecting the image in said fourth direction, asecond objective lens in the path of the image in said fourth direction,and a fifth reflecting mirror for reflecting the image to said photocellvia said grating in the final direction and producing the moire fringepattern.
 5. A device as claimed in claim 4 wherein said grating is anelongated planar grating adapted to measure the linear displacement ofthe grating.